Letterpress printing is a well-known printing process by which a rigid printing forme comprising raised portions corresponding to the pattern to be printed is inked with a relatively viscous, pasty ink, the inked pattern being thereafter applied to the paper either directly by contacting the paper and the inked printing forme or indirectly by first inking a transfer medium (or blanket) and thereafter applying this transfer medium to the paper.
Letterpress printing is to be distinguished from flexography (which printing process also falls under the category of relief printing). Unlike letterpress printing, flexography uses low viscosity inks and resilient or soft, flexible printing plates, and the pressure applied between plate cylinder and substrate is low (one commonly refers to “kiss printing” in that respect). The reader may refer to chapter 2.3 entitled “Letterpress printing”, pp. 395-408 of the Handbook of Print Media (H. Kipphan, ISBN 3-540-67326-1) for further details about letterpress printing and flexography. Flexographic printing machines are for instance disclosed in DE 29 41 521, DE 43 08 492 and EP 0 818 309.
Sheet-fed or web-fed letterpress printing machines are known in the art, among others in the field of printing of securities such as banknotes and other similar valuable documents. These machines commonly comprise a letterpress forme cylinder carrying at least one rigid letterpress printing forme having reliefs corresponding to the coloured zones to be printed, an impression cylinder which cooperates with the said forme cylinder, and an inking system for inking the forme cylinder.
In the field of security printing, letterpress printing is especially applied for the numbering of banknotes or similar valuable documents. Numbering machines for the numbering of banknotes or other valuable documents are for instance known from DE 1 486 894 and EP 0 061 795. In such numbering machines, the forme cylinder is designed as a numbering cylinder which commonly comprises a main shaft carrying a plurality of supporting discs each bearing a plurality of numbering boxes. Each numbering box typically consists of a plurality of individually rotatable numbering wheels disposed parallel to one another on a common shaft, each numbering disc comprising at its periphery a plurality of letterpress printing patterns representing alphanumeric characters or symbols. A switching mechanism is further provided to switch selected ones of the numbering wheels of each numbering box following each printing operation so that a unique alphanumeric sequence (or “serial number”) is formed by each numbering box and can be applied to a corresponding location on the printed sheets or webs. In the above numbering machines, the inking system associated to the numbering cylinder inks the surface of the various numbering wheels of each numbering box and this inked surface is applied to the printed sheets or webs to be numbered. Other examples of numbering machines and/or numbering boxes for such numbering machines are further described in DE 30 47 390, DE 36 18 488, EP 0 167 196, EP 0 718 112 and WO 2004/016433.
The above printing machines are commonly equipped with a main drive comprising a single motor. The cylinders and rollers of the printing machine are driven by means of this motor, through a system of gear wheels, pinions and/or belts, generally located on a lateral part of the machine, the so-called “drive side”. Thus the rotation of all the rollers and cylinders are synchronised. In particular, the circumferential speed of the letterpress forme cylinder or cylinders is the same as that of the various rollers of the inking system. The use of a single motor and a common gear drive system for driving all cylinders and rollers of the printing machine is accepted as a general rule in this field for avoiding all synchronisation problems.
Use of separate drives has been proposed in the context of flexographic printing machines. European patent application EP 0 818 309 for instance discloses a flexographic printing machine with three separate drives for driving respectively the inking rollers, the flexographic plate cylinders and the impression cylinder. The flexographic plate cylinders are coupled to a common drive via so-called Schmidt-couplings (or parallelogram couplings) enabling compensation of axis offsets and use of a single set of gear wheels for varying plate cylinder diameters. While separate drives are used, these drives are nevertheless controlled in such a manner that the circumferential speed of the inking rollers and flexographic plate cylinders matches that of the impression cylinder. In that respect, driving of the rollers and cylinders is also made in synchronism as in the case of a single drive.
A particularity of letterpress printing, especially letterpress printing in rotary printing machines, resides in the fact that the inking of the letterpress printing forme is not perfectly homogeneous. In particular, there is a tendency for ink to accumulate on the edge of the raised patterns of the letterpress printing forme, especially on the sides of the raised patterns which are oriented in or opposite the rotational direction of the letterpress forme cylinder (i.e. perpendicularly to the axis of rotation of the forme cylinder). As a result, the printed pattern is not perfectly homogeneous and tends to create a sort of shadow effect on one side of the printed pattern where ink has accumulated, which can be a problem in terms of printing quality. Inks which accumulates on the sides of the raised portions of the letterpress printing forme thus has to be wiped away by periodically cleaning the letterpress printing forme, which cleaning process inevitably takes some time and negatively affects the production efficiency.
A further problem resides in the fact that disassembling and mounting of the rollers of the inking system for cleaning or replacement purposes is tedious and requires disconnecting and connecting, respectively, the gear drives that couples the inking system to the other rotating parts of the printing machine.